The operation of a photoelastic modulator (PEM) type ellipsometer is described in terms of Mueller matrix elements. The phase shift (5 and the relative amplitude attenuation ratio tan * between orthogonal polarization components are contained in the Mueller matrix elements and can be obtained through combinations of measured matrix elements. A phase shift measurement accuracy of ±0.2° has been obtained without calibration. A computer-controlled implementation is described along with simple algorithms to extract the ellipsometric constants. The Mueller matrix of a magneto-optic (MO) film medium system has been used along with the Mueller matrix for a "leaky" beamsplitter to form the Mueller matrix for an ideal MO read-back system. One of the Stokes parameters is proportional to the differential detection signal commonly used in MO data detection schemes.